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Designing a Multi-Segmented Robot for Hull Climbing

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The Multi-segmented Magnetic Ro bot (MSMR) project addresses a capability gap in the intelligence, surveillance, and reconnaissance needs of the U.S. Navy visit, board, search, and seizure (VBSS); Navy SEALs; and Marine Force Reconnaissance teams. A segmented robotic platform with magnetic wheels and a minimal acoustic signature was developed that can navigate the hull, tanks, and passageways of a ship. The goal was to provide effective climbing and turning ability over and within a ferrous hull that typically features plumbing, protrusions, and indentations such as weld seams where hull plating meets. Such a robot will be able to climb the hull of a ship, provide covert perch-and-stare surveillance of the deck area, and wirelessly transmit audio/video before a search team boards. The technology is also promising for inspection of tanks, and dangerous or hard-to-reach passageways and voids in maritime vessels.

Figure 1. The chassis is the structural element of each robot module, enclosing the electronics, drive modules, and battery.
The MSMR robot system is composed of the robot modules, linkages, and magnetic wheels that provide attraction and traction with the surface being climbed. The robot modules contain the system electronics, motors, and batteries. The exterior of the robot module protects its contents from water, dust, dirt, and impact. The flexible linkages allow relative motion between robot modules so the system can turn, negotiate obstacles, and traverse around corners.

Robot Module Chassis

The chassis is the structural element of each robot module, enclosing the electronics, drive modules, and battery (see Figure 1). The first iteration of the robot module was developed to accommodate rapid changes in design by using modular mechanical interfaces for the linkage and drive module. O-rings and seals to protect internal components from the elements were not included in the first iteration; future iterations will include environmental sealing. A plastic prototyping machine was used to fabricate the robot module chassis with a polycarbonate ABS blend. Final versions of the robot module will most likely be machined from aluminum to increase strength and durability.

Magnetic Wheel

The magnetic wheel provides the attractive force between the MSMR and the surface it’s climbing. This feature allows the robot to traverse vertical and inverted surfaces, making it useful for exploring ships, shipping containers, and other ferrous environments. The primary design attributes that drive the effectiveness of a magnetic wheel are adhesion force, surface friction, acoustic signature, shock absorption, mass, cost, manufacturability, ease of assembly, and serviceability.

For a wheel to climb a surface effectively, it needs enough adhesion force to carry the weight of the MSMR so that the friction between the wheel and surface being climbed keeps the wheel from sliding. The acoustic signature of a wheel while climbing a surface is important for operations where stealth is required. Flexibility of the wheel provides survivability for the entire system under high-shock loading when the robot falls from a vertical surface. Minimizing the mass of the wheel and MSMR, in general, reduces the required magnetic forces for climbing, motor output torque, and electrical power.

The flux-plate wheel components consist of an elastomer wheel, two fluxplates, a flux-plate locator, a rigid hub, and an array of magnets oriented parallel to the central axis of the wheel. The magnets are positioned with all the north poles facing one side of the wheel, and the south poles facing the other. The elastomer wheel is made of 1-inch-thick neoprene. In addition to locating the other components of the wheel, it allows the entire assembly to flex during impact with the ground.

The outer surface of the wheel has a high coefficient of static friction, maximizing traction with hull surfaces. The rigid hub in the middle of the wheel translates torque from the output shaft of the drive motor to the wheel assembly to facilitate motion. The flux-plates direct the magnetic flux of the magnet array through the surface being climbed, providing adhesion. The flux-plate locators help keep the flux-plates centered on the elastomeric wheel. A prototype of the optimized design is 1.25" wide, 4" in diameter, and has a measured attraction force of 21 lbf.

Drive Module

The drive module provides the torque to rotate the wheels and move the robot. The main components are the motor, gearbox, output shaft, housing, motor shaft shock isolator, and bearings. The primary design considerations for the drive module were torque output, speed output, shock absorption, weight, robustness, and modularity. Required motor torque for a single motor output was 20 inch-pounds. The required speed output was 57 rpm. These values were used to select an optimal motor gearbox combination.

The development of the MMR was very dynamic, and experimentation and prototyping was used to try many different ideas. For this reason, modular designs were used for the drive modules, allowing them to be quickly moved from one robot chassis design to another. The entire drive module can be quickly detached from any chassis by removing four screws around the perimeter of the housing.

Most unmanned ground vehicles are subjected to large shock loads as the robot traverses over rough ground. Robots that climb or are thrown tend to experience even larger shock loads when they fall and hit the ground. Gearbox output shafts are often hardened and will easily break if a large radial load is experienced. For this reason, the drive module was designed to mitigate the effects of large shock loads when the robot falls onto the wheels. The output shaft is supported by two high-load bearings directly coupled to the drive module housing. If the robot falls and impacts a wheel, the radial loads are distributed through the output shaft to the drive module housing, and back to the robot chassis instead of to the gearbox output shaft. The motor shaft shock isolator allows relative motion between the output shaft and gearbox shaft during impacts that cause the output shaft to deflect, while allowing torque to be transmitted from shaft to shaft for vehicle motion.

News: Robotics, Automation & Control

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